1. Field of the Invention
The invention relates to the field of turbine engines and its subject is the control of the flow of a fluid through a rotor element depending on the rotation speed of the latter. Its subject more particularly is the cooling of a turbine rotor of a gas turbine engine.
2. Description of the Related Art
A turbojet with front fan and with two spools, for example, comprises a low-pressure spool, called the LP spool, and a high-pressure spool, called the HP spool.
By convention, in the present application, the terms “upstream” and “downstream” are defined relative to the direction of flow of the air in the turbojet. Therefore, a two-spool turbojet with front fan conventionally comprises, from upstream to downstream, a fan, an LP compressor stage, an HP compressor stage, a combustion chamber, an HP turbine stage and an LP turbine stage.
After combustion in the combustion chamber, the gas stream at very high temperature is expanded in the turbines. The elements of the HP turbine subjected to very high temperatures are swept by a cooling fluid.
With reference to FIG. 1, representing the high-pressure spool of a turbojet according to the prior art seen in axial half-section, a cooling circuit is arranged in the engine in which a stream of air, drawn upstream of the combustion chamber 1, is inserted into a cooling volume V arranged between the downstream surface of a sealing disk 3 and the upstream surface of an HP turbine rotor 5.
The sealing disk 3, the function of which is to provide the seal between the rotor and the stator upstream of the turbine, is commonly called the labyrinth disk. It takes the form of an annular part secured to a turbine disk of the HP turbine rotor 5 by its radially inner edge, called the attachment edge 33. The sealing disk 3 is prestressed axially so that its radially outer edge, called the bearing edge 32, presses against the upstream surface of the rim of the HP turbine rotor 5. The sealing disk 3 also comprises a plurality of ventilation holes 31 placed annularly and arranged to allow the insertion of a stream of cooling air between the sealing disk 3 and the HP turbine disk.
An air injector 2 draws an air stream flowing round the combustion chamber 1 and injects it into the cooling volume V via the ventilation holes 31 arranged in the sealing disk 3; the cooling air stream is then distributed in the cooling circuit of the turbine rotor 5 comprising the platforms and the blades 7 mounted on the turbine disk of the turbine rotor 5.
The ventilation holes 31 of the sealing disk 3 are arranged so as to allow an air flow rate that is sufficient to cool the HP turbine disk when the engine is running at full speed, in particular during the take-off phase of the aircraft and when the temperature of the gases is highest.
At cruising speed, the temperature of the gases is not so high and less cooling air is required. Since the flow rate of cooling air supplied by the air injector 2 is not regulated, the HP turbine disk is cooled excessively at cruising speed.
To limit the wastage of cooling air and reduce the specific fuel consumption, a cooling circuit has been proposed that connects the bottom of the combustion chamber 1 to the cooling volume V. It comprises an air flow rate regulating valve the opening of which is controlled depending on the engine speed. Such a valve would make it possible to regulate the flow rate of the cooling air stream in the cooling volume V.
However, such a device has many drawbacks. First of all, in terms of space requirement, it is difficult to arrange sufficient space in the engine to mount the regulating valve. The latter should be installed outside the engine. It would then be necessary to modify the structure of the engine by providing, for example, hollow stator air foils to allow air to be reinserted into the engine and a guidance means under the chamber in the direction of the cooling volume V. Finally, in terms of reliability, a failure of the valve could cause insufficient cooling of the HP turbine and the destruction of the engine.
Document U.S. Pat. No. 3,575,528 A1 describes a system for cooling rotor turbines comprising a bimetallic ring forming a valve arranged to block an air passage.